Use of a particular coiling temperature in the production of electrical steel sheet



United States Patent USE OF A PARTICULAR C(HLENG TEMPERATURE I IV THE PRQDUCTEON G1 ELECTRECAL STEEL SHEET Robert D. Holhein, Buifalo Township, Butler County,

Edward B. Stanley, Washington Township, Westmoreland County, and Harry G. Stout, Peters Township,

Washington County, Pa., assignors to United States Steel Corporation, a corporation of New Jersey N0 Drawing. Filed Feb. 26, 1953, Ser. No. 261,203

7 Claims. (Cl. 148-120) This inventionrelates to improvements in the treatment of low-carbon steel to improve the electrical properties thereof.

It has heretofore been proposed to cold-reduce, decarburize, and temper roll low-carbon rimmed steel to render it suitable for use in magnetic cores of low cost electrical equipment. Such material is frequently stressrelief annealed by the electrical equipment manufacturer after laminations are punched and assembled into core members for electrical equipment. The steel is decarburized by suitable annealing procedures such as box annealing with the hot mill scale on the steel surfaces or by exposing the steel surfaces to suitable decarburizing atmospheres. While steel so produced has desirable properties, the permeability and core loss are such that they limit the usefulness thereof.

It is accordingly an object of the present invention to provide an improved method of producing low-carbon steel for electrical uses.

It is a further object of this invention to improve the permeability and core loss of low-carbon steelin an efficient and economical manner.

We have found that a highly desirable product, from a magnetic-property standpoint, can be produced from low-carbon rimmed or capped steel by controlling coiling temperature after hot rolling in combination with a controlled open-coil annealing practice after cold reduction to decarburize and anneal followed by roller leveling or by temper rolling up to 10% extension. Low-carbon capped or rimmed steel of the following composition may be used:

Percent Carbon 0.03 to 0.14 Manganese 0.38 to 0.76 Phosphorus 0.008 to 0.15 Sulphur maX 0.03 Silicon max 0.020

Percent Carbon 0.03 to 0.14 Manganese 0.45 to 0.76 Phosphorus 0.10 to 0.15 Sulphur 0.018 to 0.028 Silicon max 0.020 Copper "max" 0.15 Nickel m 0.15 Chromium max 0. 100 Molybdenum max 0.050

with the balance essentially iron.

F. instead of cooling to customary temperature of 1150 F. or lower before coiling. The coils are then open-coil annealed as hereinafter described. Open-coil anneal is a term applied to annealing wherein the wraps of the coil are positively separated a determined amount by wire or the like spacers to permit access of controlled atmosphere to all of the surfaces of the strip. Thus the rate of heating and surface reactions can be closely controlled. In the cycle of the present invention, the steel coil after being .opened" is heated in a closed furnace to an annealing temperature within the ranges of 1350 to 1550 F. at a heating rate such that the steel is within the range of 1150 to 1300 F. for sufiicient time to reduce the carbon content to 008% maximum and preferably to less than 005%. During the annealing cycle, an atmosphere of hydrogen and nitrogen is maintained in the furnace; HNX gas containing about 8% hydrogen and 92% nitrogen is satisfactory. When the temperature of the steel reaches about 1150 F. water vapor, such as steam, is injected into the atmosphere to raise the dew point from its preliminary controlled low level of less than 0 F. and preferably be low about -40 F. to about F. After the temperature reaches about 1300 F. and decarburizing is substantially completed, the dew point is again lowered to below 0 F. Heating rates of to 442 F. per hour depending on the thickness of the strip, which is usually in the range of .0185 to .025 inch, may be used to bring the temperature to about 1300 F. Thereafter the dew point is lowered as indicated and the temperature of the steel is raised to a grain coarsening annealing temperature of 1500 to 1550 F. and preferably at about 1525 F. at a rate of 18 to F. per hour and held at this temperature for a time of 2 to 6 hours. Thereafter the steel is cooled to about 200 F. at a rate of 128 to 253 F. per hour. Table I based on 0.025-inch-thick material shows the improvement in core loss resulting from increasing the hot-mill coiling temperature from less than 1150 F. to within the range of 1200 to 1300 F., increasing anneal ing temperature within the range of 1450 to 1550 F. after decarburizing and the effect of increased amounts of temper rolling from. 22% maximum or /2 to 1% to 5 to 10%.

Table I to 1% 5 to 10% Grain Max. Temper Temper Size After Box Temper Roll Roll pen- Annealing Coiling Roll Core Core Coil Temp, Temp, F. Core Loss, Loss, Anneal, F. Loss, w./lb./ w.[lb./ ASTM w./lb./ 15 kg. 15 kg No. 15 kg.

1 300 {1,050 to 1,150.... 6. 04 4. 98 4. 38 6. 2 1,200 to 1.300 6.08 4. 92 4.08 7. 1 1 500 {1,050 to 1,150... 5. 51 4. 88 4. 30 4. 8 1,200 to 1,300.--. 5. 39 4. 76 4 02 4. 6

1 This treatment conducted after open-coil annealing. 2 Values alter a stress-relief anneal at 1,450" F.

Norn.0arbon content after open-coil annealing less than 008% and generally less than 0.005%. V

The data of Table II shows the permeability resulting from the increasing amounts of temper rolling. As would be expected, the permeability was somewhat impaired by the increased annealing temperature, but this is ofiset by increasing amounts of temper rolling above 1%.

1 This treatment conducted after open-coil annealing. 2 Values after a stress-relief anneal at 1450" F.

As a specific example of the practice of our invention, steel of the following composition 0.04% carbon, 0.47% manganese, 0.14% phosphorus, 0.020% sulphur, balance substantially iron, was hot-rolled to a thickness of 0.080 inch, coiled at a temperature of 1240 F., and then pickled in a solution of 8 to 23% sulphuric acid. The steel was cold reduced to 10% above a final gage of 0.025 inch. The steel was then open-coil annealed in an atmosphere of hydrogen and nitrogen in amounts of 9% and 91%, respectively. The steel during annealing was initially heated at a rate of 200 F. per hour for five hours to a temperature of 1150 F Thereafter, steam was injected into the annealing furnace to raise the dew point from the prevailing 40 F. to +60 F. and heating was continued for one hour to a temperature of 1300 F. The dew point was then lowered to F. and the temperature of the steel was raised to 1500 F. at a heating rate of 75 F. per hour and held at this temperature for three hours. The steel was then cooled to about 200 F. at a rate of 200 F. per hour. The steel was thereafter temper rolled to achieve a reduction in gage of 10%. The temper rolled material was then stress-relief annealed at 1450 F. Upon testing, the steel exhibited a core loss value of 4.03 w./lb./ kg. and an A.-C. permeability of 2160 at 15 kilogausses. The grain size after open-coil annealing was No. 5 ASTM and 0 to 3 after stress-relief annealing.

While the temperature and time of decarburizing may be varied somewhat, it is essential that the carbon content be reduced to less than 008% and preferably to less than .005%. The temperature and time to produce grain coarsening after decarburization may also be varied depending on the level of final magnetic properties desired. A two-fold or greater increase in grain size can be obtained by increasing the annealing temperature from about 1300 F. to a temperature of about 1525 F. For practical reasons and the fact that the material may become distorted physically at high annealing temperatures, we prefer, but do not limit, the grain-coarsening temperature to about 1525 F.

Material that is temper rolled to a maximum of /2% is ordinarily not stress-relief annealed after punching or forming. However, material that is temper rolled over /z% extension is ordinarily stress-relief annealed after punching or forming. Thus the data in Tables I and II for material temper rolled over /2% is for stress-relief material and for material temper rolled less than /2% is in the temper rolled condition.

The data clearly shows that for low-carbon rimmed or capped steel containing over about .10% phosphorus and more than about .45% manganese sheet material, coiled at the higher temperaturatures, decarburized either by holding at temperature or heating through the range of 1150 to 1300 F. and thereafter grain coarsened by annealing at about 1525 F. has desirable properties after temper rolling less than Also such material will have good electrical properties after stress-relief annealing if temper rolled to between .5 and 10% elongation and preferably to between 5 and 10% elongation. Such Percent Carbon .03 to .14 Manganese .38 to .76 Phosphorus .008 to .15 Sulphur max .03 Silicon ..max .02

with the balance iron and residual amounts of other elements, hot rolling said steel to strip, coiling the strip at a temperature between 1200 and 1300 11, cold reducing said strip, decarburizing said strip by heating in the range of 1150 to 1300 F. in a moist atmosphere for sufiicient time to reduce the carbon content to less than .008, then raising the temperature thereof to within the range of 1450 to 1550 F. to increase the grain size thereof and thereafter temper rolling said strip to elongate it not more than 10%.

2. A method of producing low-carbon steel having improved electrical properties comprising melting steel containing Percent Carbon .03 to .14 Manganese .38 to .76 Phosphorus .008 to .15 Sulphur Inax .03 Silicon max.. .02

with the balance iron and residual amounts of other elements, hot rolling said steel to strip, coiling the strip at a temperature between 1200 and 1300 F., cold reducing said strip, decarburizing said strip by heating in the range of 1150 to 1300 F. in a moist atmosphere for sufficient time to reduce the carbon content to less than 008%, then raising the temperature thereof to within the range of 1500 to 1550 F. for between 2 to 6 hours to increase the grain size thereof, temper rolling said strip to elongate it between .5 and 10% and thereafter stress-relief annealing it.

3. A method of producing low-carbon steel having improved electrical properties comprising melting steel containing Percent Carbon .03 to .14 Manganese .38 to .76 Phosphorus .008 to .15 Sulphur "max" .03 Silicon max .02

with the balance iron and residual amounts of other elements hot rolling said steel to strip, coiling the strip at a temperature between 1200 and 1300 F., cold reducing said strip, decarburizing said strip by heating in the range of 1150 to 1300 F. in a moist atmosphere for sufiicient time to reduce the carbon content to less than 008%, raising the temperature to within the range of 1500 to 1550 F. for between 2 to 6 hours to increase the grain size thereof, temper rolling said strip to elongate it between 5 and 10% and thereafter stress-relief annealing it.

4. A method of producing low-carbon steel having improved electrical properties comprising melting steel containing with the balance essentially iron, hot rolling said steel to strip, coiling the strip at a temperature between 1200 and 1300 F., cold reducing said strip, decarburizing said strip by heating in the range of 1150 to 1300 F. in a moist atmosphere for sufiicient time to reduce the carbon content to less than .055%, raising the temperature to about 1525 F. for 2 to 6 hours to substantially increase the grain size thereof and thereafter temper rolling said strip to elongate it less than 10%.

5. A method of producing low-carbon steel having improved electrical properties comprising melting steel containing with the balance substantially iron, hot rolling said steel to strip, coiling the strip at a temperature between 1200 and 1300 F., cold reducing said strip, decarburizing said strip by heating in the range of 1150 to 1300 F. in a moist atmosphere for suflicient time to reduce the carbon content to less than .005 raising the temperature to about 1525 F. for 2 to 6 hours to substantially inproved electrical properties comprising melting steel containing Percent Carbon .03 to .14 Manganese .45 to .76 Phosphorus .10 to .15 Sulphur m .028 Silicon max .02 Copper max .15 Nickel max .15 Chromium max .10 Molybdenum max .05

with the balance substantially iron, hot rolling said steel to strip, coiling the strip at a temperaure between 1200 and 1300 F., cold reducing said strip, decarburizing said strip by heating in the range of 1150 to 1300 F. in a moist atmosphere for sutficient time to reduce the carbon content to less than .005%, raising the temperature to about 1525 F. for 2 to 6 'hours to substantially increase the grain size thereof, temper rolling said strip to elongate it between 5 and 10% and thereafter stress-relief annealing it at a temperature of about 1450 F.

7. A method of producing low-carbon steel having improved electrical properties comprising melting steel containing with the balance iron and residual amounts of other elements, hot rolling said steel to strip, coiling the strip at a temperature between 1200 and 1300 F., cold reducing said strip, reducing the carbon content of said strip to less than 008%, coarsening the grains thereof and thereafter temper rolling said strip to elongate it not more than 10% References Cited by the Examiner UNITED STATES PATENTS 2,672,429 3/54 Malloy 148-120 FOREIGN PATENTS 642,689 6/62 Canada.

DAVID L. RECK, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,188,250 June 8, 1965 Robert D. Holbein et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 5, line 18, for ".055%" read .005%

Signed and sealed this 26th day of October 1965.

SEAL) ZRNEST W. SWIDER EDWARD J. BRENNER Ittesting Officer Commissioner of Patents 

1. A METHOD OF PRODUCING LOW-CARBN STEEL HAVING IMPROVED ELECTRICAL PROPERTIES COMPRISING MELTING STEEL CONTAINING 